Optical films generally obtain optical properties through a combination of choosing materials having differing refractive indices and forming the materials in a spatial relationship that results in the desired optical effect when the films interact with light.
Optical polymer films are films that exhibit certain desirable optical effects. Specifically, these films include polymer films that are designed to reflect, transmit, absorb or refract light upon exposure to a specific band of electromagnetic energy. These optical polymer films can be made with a variety of properties. One class of optical films will lose significant optical performance if exposed to an excessive amount of heat. This class achieves much of its optical properties through a multilayer or blend construction of at least two different polymers having different indices of refraction. This difference in refractive indices may be enhanced through an orientation or stretching process if at least one of the polymeric materials is capable of stress-induced birefringence. Such a process thins related polymer regions to a thickness that allows for interaction with desired ranges of wavelengths in specified ways. The regions may be in the form of thin layers of different films or thin discontinuous polymer regions within a second polymer matrix. Exposure to excessive heat may cause mixing of the discreet polymer layers, and in the case of oriented multilayer or blend films tends to relax the ordered nature caused by the orientation, thus degrading the film's optical performance.
This class of oriented films includes multilayered films and films composed of blends of two or more polymeric materials. Multilayer films provide reflective and transmissive properties through a multiplicity of layers that each have a thickness on the order of a fraction of the distance corresponding to a wavelength of light, and are useful in reflective applications. Examples of this type of film include polarizers, visible and infrared mirrors, and color films such as those described in Patent Publications WO 95/17303, WO 96/19347 and WO 97/01440, and in U.S. Pat. No. 5,103,337 (Schrenk), U.S. Pat. No. 5,122,905 (Wheatley et al), U.S. Pat. No. 5,122,906 (Wheatley), U.S. Pat. No. 5,126,880 (Wheatley), U.S. Pat. No. 5,217,794 (Schrenk), U.S. Pat. No. 5,233,465 (Schrenk), U.S. Pat. No. 5,262,894 (Wheatley), U.S. Pat. No. 5,278,694 (Wheatley) U.S. Pat. No. 5,339,198 (Wheatley), U.S. Pat. No. 5,360,659 (Arends), U.S. Pat. No. 5,448,404 (Schrenk), U.S. Pat. No. 5,486,949 (Schrenk) U.S. Pat. No. 4,162,343 (Wilcox), U.S. Pat. No. 5,089,318 (Shetty), U.S. Pat. No. 5,154,765 (Armanini). U.S. Pat. No. 3,711,176 (Alfrey, Jr. et al.); and Reissued U.S. Pat. No. RE 31,780 (Cooper) and U.S. Pat. No. RE 34,605 (Schrenk). Blend constructions obtain reflective and transmissive properties from the presence of discontinuous polymeric regions having a cross-sectional diameter perpendicular to the major axis that is on the order of a fraction of the distance corresponding to a wavelength of light. Such films may also obtain the desired optical properties through orientation. Examples of such films include blend mirrors and polarizers as described in Patent Publications WO 97/32224, filed application having U.S. Ser. No. 09/006455, and U.S. Pat. No. 5,751,388 (Larson).
A second class of optical films is relatively impervious to the effects of excessive heat. This class achieves much of its optical property from rigid replicated prismatic shapes on a surface of a flexible polymer layer. These ordered shapes provide most of the reflective optical properties of the film and are usually cured such that they are more heat resistant than the supporting polymer layer. The optical characteristics of the cured shapes do not change easily with heat. These films may be used in a transmissive, reflective, or refractive mode depending on the application. This class of films includes, for example, ordered reflective cube-corner sheeting such as those described in U.S. Pat. No. 5,450,235 (Smith et al), U.S. Pat. No. 5,691,846 (Benson et al), U.S. Pat. No. 5,614,286 (Bacon, et al) and U.S. Pat. No. 5,763,049 (Frey et al).
Embossment of the surface of an optical film can be used to manipulate its optical or mechanical properties. Physical embossing, i.e., achieving a contoured surface by pressing the surface of an optical film against a tooled surface, generally results in shallow contours because of the hardness of typical polymeric materials that are used. Thermal embossing, i.e., passing the optical film surface in pressed contact with a heated tooled surface, results in patterns having deeper contour or depth. However, the heat transfer between a heated surface and the contacting film is so efficient that the film typically is heated to near its melting point for most of the contact time, often destroying spatial relationships within the film and adversely affecting its optical properties. The controlled thickness of oriented film layers can become disordered and the ordered alignment of fibrous discontinuous regions can become unaligned. While degradation of the optical properties of cured reflective films generally is of less concern, these films too cannot be embossed easily with hot tooled surfaces. Because cured reflective films have large heat sink capacities, production speeds used to manufacture such films are limited.
Thus, there is a need to thermally emboss optical polymer films without degrading optical properties and to do so at faster speeds.